Microelectromechanical (“MEMS”) microphones are currently supplied with a fixed DC bias voltage between the diaphragm and backplate structures during normal operation. Under microphone fault conditions in connection with a so-called diaphragm collapse, a certain manipulation of the DC bias voltage to remove or decrease attractive electrostatic forces between the diaphragm and backplate has been proposed and published in EP 1 599 067 A2.
US 2006/062406 A1 discloses a condenser microphone comprising a programmable DC bias voltage for a microphone condenser transducer and a memory for storing a set value of the DC bias voltage. WO 01/78446 A1 discloses an electret microphone comprising a variable sensitivity/variable gain circuit coupled between the electret transducer and a buffer amplifier.
Other references related to calibrated microphone systems and methods are: U.S. Pat. No. 4,631,749, U.S. Pat. No. 5,051,799, U.S. Pat. No. 5,029,215, US 2003/0198354 A1, and US 2005/0175190 A1.
A significant problem in producing MEMS condenser microphones with high yield is that the compliance or tension of the MEMS microphone diaphragm varies according to a number of manufacturing parameters that are difficult to accurately control. The absolute values of physical or mechanical parameters from silicon wafers (e.g. mechanical stiffness, electric resistance, transistor trans-conductance) may easily vary by +/−20% or more. This is a significant disadvantage for well-controlled MEMS microphone fabrication.
Other physical parameters of a MEMS microphone also vary, e.g. diaphragm area, air gap height, i.e. the distance between the diaphragm and the back plate. Compared to a standard “macroscopic” microphone, in which the air gap height is larger than 30 or 50 μm, the air gap height in MEMS transducers is typically 5-10 μm or even smaller. The small dimensions of MEMS microphones impose severe limitations on how a DC bias voltage can be adjusted to compensate for a non-nominal acoustic sensitivity. Adjusting, the DC bias voltage to a high value may cause the collapse threshold, in terms of dB SPL, to move to an unacceptable low value.
The influence of varying parameters of electrical components encountered in the manufacture process of integrated semiconductor circuits, such as CMOS circuits, is usually less significant to the performance and uniformity of MEMS microphones. However, a certain influence on performance parameters such as amplifier gain and impedance remains. This influence is particularly difficult to eliminate in high volume and low-cost MEMS microphones where low-complexity amplifier topologies are essential to keep die area, and thereby cost, low. Consequently, it would be advantageous to compensate for these performance parameter variations.